The deeper water masses are classified into following categories

The deeper water masses are classified into two categories: (1) the intermediate water masses, and (2) the deep and bottom water masses. The intermediate waters, as they are called, are less dense than the bottom waters, so that they can sink to intermediate depths only.

These water masses are formed in all the major oceans of the world in high latitudes. According to Sverdrup, at the Antarctic Convergence, water of low salinity and low temperature sinks. Part of this sinking water returns towards the south in some areas at the intermediate depth of a few hundred meters.

However, the greater part of this water mass moves towards the north forming what is called the Antarctic Intermediate water. This type of water is present in all the oceans. They can be traced upto great distances from the Antarctic region.

1. This intermediate water mass is formed just north of the Antarctic Convergence:

It is here that water sinks and forms the Antarctic Intermediate water. The temperature in this water mass varies between 2.2°C and 7°C. The salinity is low – (34.1-34.6%o). This water spreads towards the north.

While sinking this water mixes with the surrounding water with the result that with increasing distance from the Convergence the characteristic T-S relation of this water undergoes a change. Its upper and lower boundaries are not clearly marked. ‘The core method’ ^[1] introduced by Wust is applied to determine the results of the mixing.

This method refers to the examining of the temperature and salinity at the core of the intermediate water to ascertain the depth of the core. Besides the South Atlantic Ocean where it is found below the Central water mass, it is also traced in the North Atlantic just north of the Equator.

2. The North Atlantic Intermediate Water mass:

The North Atlantic Intermediate water mass comprises three different types of water masses, (i) Antarctic Intermediate water mass which originated in the South Atlantic Ocean. When it enters into the North Atlantic Ocean, it mixes with the water masses of this ocean (ii) Besides, another type of intermediate water with a temperature of 3.5°C and salinity of 34.88%o, represents the Arctic Intermediate Water mass. (iii) The Mediterranean Intermediate water, which enters into the North Atlantic Ocean through the strait of Gibralter.

This last water mass is characterized by relatively high temperature (11.9°C) and high salinity (36.50%o). This water mass is not formed in the open ocean but in one of the adjacent seas. It covers a larger area than the other intermediate waters.

3. The Pacific Intermediate Water mass:

This water mass is formed at about 40°N lat. and it is overlain by the central water mass of the North Pacific Ocean. Salinity is rather very low in it. This water is found at depths of 600 to 800 m in the north – western part of the Pacific Ocean.

However, in the vicinity of the Equator it bifurcates into two layers. One layer is formed at 200m depth, while the other at 900m. In these layers the oxygen is at its minimum in the North Pacific.

The low oxygen content is found 400 to 500 m below the waters with very low salinity. Off the American coast at the depth of less than1000m, the oxygen values are still lower. This water sometimes up-wells to the surface.

4. The Mediterranean Water mass:

The Mediterranean water mass is relatively dense due to its very saline water. While passing through the Straits of Gibraltar its salinity is more than 37%o. Its temperature reaches 13°C. Intense mixing takes place in the straits.

These figures of salinity and temperature are largely modified from those characteristics of the deep water of the Mediterranean Sea. It is to be kept in mind that the Mediterranean water mass exercises a great influence on the deep water of the Atlantic by adding huge quantities of water of high salinity.

The surface water which sinks to a depth of 100 to 200m has high oxygen content. However, the oxygen values of the intermediate water are relatively higher in the eastern part of the Mediterranean where this water is formed.

However, then it decreases towards the west. The transition layer has an oxygen minimum. It is more pronounced in the eastern than in the western parts. The deep water is characterized by relatively higher oxygen content.

The Mediterranean Sea plays an important role in the deep-water circulation of the Atlantic Ocean because of the huge amount of saline water produced there. The high salinity of the Atlantic deep-water can be traced even upto the southern tip of Africa.

5. The North Atlantic Deep and Bottom water:

As the name indicates, this water mass can be conveniently divided into two types: the deep water and the bottom water, which have different origins. However, the deep and bottom water masses are characterized by homogeneity all over the Atlantic Ocean.

The deep water originates in the extreme northern part of the North Atlantic Ocean, whereas the bottom water is somewhat mixed with the bottom water of Antarctic origin. The deep water mass in the North Atlantic is found in the region, south of Greenland.

Here the warm and saline water of the North Atlantic mixes with the cold water of the Labrador Current and the East Greenland Current. This mixed water mass is cooler in winter so that its density is increased.

Thus, being denser it sinks to depths exceeding 1000m. It has temperature ranging from 2.8°C to 3.3°C, and its salinity varies between 34.9%o and 34.96%o.

It is worth remembering that the formation of the deep and bottom water mass takes place during the winter months, since in this season alone the water is cold enough to attain high density to sink to the bottom of the North Atlantic.

6. The Deep Water of the Pacific Ocean:

In the North Pacific there is no source of deep water. According to Sverdrup, the Pacific Deep water is of Atlantic and Indian origin, but has become so much diluted by admixture of intermediate and bottom water that the salinity maximum has disappeared.

“In the South Pacific, the intermediate and bottom water flow to the north, while the deep water flows to the south. It is to be kept in mind that the aforementioned north-south circulation is superimposed upon a general flow from west to east”.

In the South Pacific deep water there is a gradual increase of salinity which remains constant below 2500 to 3000 m. The oxygen content in the deeper layers is lower in the North Pacific than the South Pacific.

7. The Antarctic Bottom Water mass:

This is undoubtedly the densest of all the water masses, the most extreme type of bottom water is formed in the Weddel Sea area, where below a depth of about 400m the temperature is roughly about -0.4°C, and the salinity is about 34.66%o. This extremely cold water is the result of the very cold water at the surface.

Because of freezing its salinity registers an increase with the result that water density is further increased. As a matter of fact, the Antarctic Bottom Water is formed as an equal mixture of Circumpolar Water and the shelf water.

The shelf water has a temperature of -1.9°C and a salinity of about 34.62%o. The Circumpolar Water has a temperature of 0.5°C and a salinity of 34.68%o. Resultant bottom water below 4,000 m is characterized by a salinity of 34.66%o and a temperature of -0.4°C.

Its density is 27.86. It is to be kept in mind that the freezing of sea water in the formation of the Antarctic Bottom Water mass plays the most important part.

There is one very interesting point which should not be lost sight of. This is concerning the shelf water which plays a significant role in the formation of the bottom water. The shelf water, the salinity of which may be increased up to 34.62% or slightly higher by freezing of ice has the highest density.

Owing to these characteristics, the shelf water sinks down along the continental slope. During this process it is mixed with the circumpolar water of a little lower density. It is in this way that the bottom water is formed which is denser than the deep water.

As stated above, the major part of the Bottom Water is formed in the Weddell Sea. But bottom water is also formed south of the Indian Ocean between 30°E and 140° E long.

However, to the south of the Pacific Ocean no such formation takes place. The abovementioned characteristics of the bottom water exercise greater influence on the deep water circulation of all the three major oceans of the world.